The purposes of the formation of a chromate conversion coating on the surface of aluminum are to provide corrosion resistance, improve adhesion of coatings and for esthetic reasons. The conversion coating improves the adhesion of coating layers such as paints, inks, lacquers and plastic coatings. A chromate conversion coating is typically provided by contacting aluminum with an aqueous composition containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. Growing concerns exist regarding the pollution effects of the chromate and phosphate discharged into rivers and waterways by such processes. Because of high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate conversion processes require extensive waste treatment procedures to control their discharge. In addition, the disposal of the solid sludge from such waste treatment procedures is a significant problem.
Attempts have been made to produce an acceptable chromate-free conversion coating for aluminum. Chromate-free pretreatment coatings based upon complex fluoacids and polyacrylic acids are known in the art, however they have not enjoyed widespread commercial acceptance. U.S. Pat. No. 4,191,596 which issued to Dollman et al., discloses a composition for coating aluminum which comprises a polyacrylic acid and H.sub.2 ZrF.sub.6, H.sub.2 TiF.sub.6 or H.sub.2 SiF.sub.6. The '596 disclosure is limited to a water soluble polyacrylic acid or water dispersible emulsions of polyacrylic acid esters in combination with the described metal acids at a pH of less than about 3.5.
U.S. Pat. No. 4,921,552 which issued to Sander et al., discloses a non-chromate coating for aluminum which is dried in place which forms a coating having a weight from about 6 to 25 milligrams per square foot. The aqueous coating composition consists essentially of more than 8 grams per liter dihydrohexafluozirconic acid, more than 10 grams per liter of water soluble acrylic acid and homopolymers thereof and more than 0.17 grams per liter hydrofluoric acid. The disclosure notes that it was believed that copolymers of acrylic acid would also be effective, however, no examples were given.
U.S. Pat. No. 4,136,073 which issued to Muro et al., discloses a composition and process for the pretreatment of aluminum surfaces using an aqueous acidic bath containing a stable organic film forming polymer and a soluble titanium compound. The disclosed polymers include vinyl polymers and copolymers derived from monomers such as vinyl acetate, vinylidene chloride, vinyl chloride; acrylic polymers derived from monomers such as acrylic acid, methacrylic acid, acrylic esters, methacrylic esters and the like; aminoalkyl, epoxy, urethane-polyester, styrene and olefin polymers and copolymers; and natural and synthetic rubbers.
The use of allyl ether copolymers in non-analogous arts such as dust control, dispersants and water treatment is known. Japanese patent publication SH056-155692 entitled Method of Collecting Dust discloses the use of acrylic acid/polyethylene glycol monoallyl ether copolymers to treat the recirculating water in an aqueous dust collection system. U.S. Pat. No. 4,500,693 which issued to Takehara et al., discloses the use of copolymers composed of a (meth)acrylic acid and an allylic ether monomer which are useful as scale preventing agents in cooling water systems and wet dust collection systems, aqueous slurry dispersants in inorganic pigments, cement dispersants, and builders in detergents.
U.S. Pat. No. 4,471,100 which issued to Tsubakimoto et al., disloses a copolymer of maleic acid and polyethyleneglycol ether and its use as a cement dispersant, pigment dispersant, chelating agents and scale inhibitor.
U.S. Pat. Nos. 4,872,995 and 4,913,822 to Chen et al., and U.S. Pat. Nos. 4,861,429 and 4,895,622 to Barnett et al., disclose methods of using acrylic acid/polyethyleneglycol allyl ether copolymers in aqueous systems such as cooling water systems and paper making systems, as felt conditioners or to inhibit calcium oxalate deposition.